I'm getting ready to buy Rybka 3 with the chessbase interface, and up until now, I've always owned the single processor versions of the yearly updates such as Shredder 9 and Fritz 11. Can anyone tell me just exactly what do the multi-processor versions do that the single processors can't? I know the mutli versions like Deep Rybka 3 and Deep Fritz 11 analyze much faster, but are there any other benefits?  

Nope just faster, but that means alot in playing strength and analisis. The faster the engine is the more variations it can calculate the more acurate the analisis of a game it will make with less time. It's worth the extra cash in any case.

As a technician, I would just like to point out the difference between multiprocessor systems and multicore systems. Most processors sold today are dual or quad core, meaning they have 2 or 4 processor cores embedded into the same physical processor. These cores act independently, carry out calculations etc. simultanousely but share perhaps some of the L2 or L3 cache memory. If you have a multi-core processor, you can get a real performance boost using a multithreaded chess program.

Multiprocessor systems are systems built with more than one physical processor, and very rare among average (and even high-performance) consumers.

Thanks for the clarification guys. I currently do not have a multi-core system, but I do have a multiprocessor system (Pentium 4CPU, 2.8 GHz, 2.79 GHz, 512MB of RAM). Would these qualifications still make it worth it to have a multiprocessor program?

Yes. As long as you have more than one processor core you have a multiprocessor system. In your case you have two physical processors with one core - rarer than having one physical cpu with 2 or more cores, but it definitely counts.

Next to that, some Pentium IV's supported HyperThreading, which means they can in certain cases work like a multicore system. Not as fast as real multicore systems though.

Good luck with the chessplaying fish.

 I agree with all you say, with one excepotion and that is if you can build your own computer or know someone that can help you put it together, you will need to spend about $ 2000 US or less, even with a 2.6+ processor quad core and 4 Gb memory.

Just thought I mention it, as this is what I did.